Net Snowpack Accumulation and Ablation

Hydrology 2014, 1, 1-19; doi:10.3390/hydrology1010001 OPEN ACCESS

hydrology ISSN 2306-5338 www.mdpi.com/journal/hydrology Article

Net Snowpack Accumulation and Ablation Characteristics in the Inland Temperate Rainforest of the Upper Fraser River Basin, Canada Stephen J. Déry 1,*, Heidi K. Knudsvig 2, Marco A. Hernández-Henríquez 1 and Darwyn S. Coxson 3 1

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Environmental Science and Engineering Program, University of Northern British Columbia, Prince George, BC V2N 4Z9, Canada; E-Mail: [email protected] Natural Resources and Environmental Studies Program, University of Northern British Columbia, Prince George, BC V2N 4Z9, Canada; E-Mail: [email protected] Ecosystem Science and Management Program, University of Northern British Columbia, Prince George, BC V2N 4Z9, Canada; E-Mail: [email protected]

* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +1-250-960-5193; Fax: +1-250-960-5845. Received: 7 December 2013; in revised form: 19 March 2014 / Accepted: 3 April 2014 / Published: 28 May 2014

Abstract: This study examines the net snow accumulation and ablation characteristics and trends in the Inland Temperate Rainforest (ITR) of the Upper Fraser River Basin, British Columbia (BC), Canada. It intends to establish whether elevation and/or air temperature play(s) a dominant role in hydrological year peak snow water equivalent (SWE) and whether regional patterns emerge in the interannual variability in peak accumulation. To that end, SWE and air temperature data from seven snow pillow sites in the Upper Fraser River Basin at elevations ranging from 1118 to 1847 m above sea level are analyzed to infer snowpack characteristics and trends for hydrological years 1969–2012, with 2005–2012 being the actual period of data overlap. Average peak SWE ranges from 391.3 mm at Barkerville, BC on 16 April to 924.4 mm at Hedrick Lake, BC on 27 April. Snow cover duration lasts 206–258 days, with snow onset dates from mid-October to early November and snow off dates from late May to early July. Statistically-significant (p ≤ 0.05) cross correlations exist between peak SWE at nearly all sites, indicating regional coherence in seasonal synoptic activity across the study area. However, the lack of relationships between peak SWE and elevation as well as air temperature parameters indicate that mesoscale to local processes lead to distinct snow accumulation and ablation patterns at each site. Four

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sites with the longest records exhibit no trend in peak SWE values between 1990 and 2012. Changes to snowpack regimes may pose a threat to the productivity and immense biodiversity supported by the ancient western red cedar and hemlock stands growing in the wet toe slopes of the ITR. Thus, it is imperative that continued monitoring of snowpack conditions remains a top priority in the Upper Fraser River Basin, allowing for a better understanding of ecosystem changes in a warming climate. Keywords: snowpack; snow accumulation; snow ablation; Inland Temperate Rainforest; climate change; ecohydrology; Upper Fraser River Basin; British Columbia

1. Introduction Snow forms an important component of the hydrological cycle and climate of high latitude and mountainous regions (e.g., [1,2]). As an example, the Upper Fraser River Basin of British Columbia (BC), Canada, is a snow-dominated system with snowmelt resulting in an annual pulse of freshwater each spring and early summer that periodically leads to flooding. This region is dominated by abundant wintertime snowfall throughout northern BC’s mountainous terrain and vast forests. It is also the site of the Inland Temperate Rainforest (ITR), an ecosystem unique to BC that is characterized by its continentality and anomalously humid climate. The continentality is depicted by the same weather systems and precipitation patterns that nourish the coastal rainforests of BC but in a cooler climate regime, creating a secondary zone of high precipitation as they cross the interior mountain ranges [3]. Confined to the wettest subzones of the Interior Cedar-Hemlock zone (i.e., biogeoclimatic zones ICHwk and ICHvk [4]), the ITR experiences plentiful snowmelt during late spring that is followed by ample rainfall during the height of the growing season [5,6]. The ITR shelters old-growth forests, with some trees surpassing 1000 years of age, which are largely found in valley-bottom to mid-slope positions on the windward slope of the interior mountain ranges, between latitudes 50°N and 54°N [3]. In particular, cedars and hemlocks are abundant at the toe slopes, whereas old-growth Engelmann spruce and subalpine fir (ESSF) thrive at higher elevations in the subalpine above 1500 m above sea level (a.s.l.) [3,6]. The ITR is ≈700 km inland from the coast of the Pacific Ocean and differs from BC’s coastal rainforests because most of its annual precipitation falls as snow (especially at elevations above 1000 m a.s.l.). Snowpacks are moderate to deep in the ITR, with the wettest portions accumulating up to 2 m of settled snow in mid to late winter and much deeper accumulation occurring at higher elevations in the ESSF zone [3]. Deep snowpacks extend the snowmelt period over the summer months, which are thought to gradually replenish soil moisture in wet toe slope topographic positions, where ancient western redcedar and hemlock stands are found [3]. In addition, the abundant snowmelt infiltration in this region sustains groundwater supply and minimizes soil moisture deficit during dry summer periods, making stands less susceptible to fires and insect mortality [6]. Despite the important role of snow in its hydrology, no study has explored net snowpack accumulation and ablation characteristics of the Upper Fraser River Basin. Previous research on western North American snowpack characteristics focused on the Sierra Nevada [7], Colorado Rockies [8], and the Pacific Northwest [9] including the headwaters of the Columbia River Basin in BC [10]. In

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western North America, climate change is one of the main drivers of declining mountain snowpacks; however, land use modifications and other factors also impact winter snowpacks [11]. A regional analysis of snowpack metrics for the interior western USA indicated widespread decreases in the duration of snow cover throughout the intermountain west, reduced maximum Snow Water Equivalent (SWE), and faster melts in the Colorado River and Rio Grande Basins in response to widespread warming [12]. Forest harvest practices, such as partial or clear cutting, can result in greater accumulation of total SWE in the winter snowpack [13,14]. Mountain pine beetle killed forests exhibit higher winter snowpack accumulation [15], more rapid snowmelt [16], and increased sublimation [17]. Following wild fires, canopy loss can increase net winter season snow sublimation and post-burn areas may experience increased winter ablation that can lead to reduced snow water inputs relative to healthy forests [18]. Danard and Murty [19] observed declining snowpack accumulations from 1966 to 1989 along with warming trends in the Fraser and Skeena River Basins of BC. Sites along the middle Fraser River were found to experience sharp declines in final day of the month snow depths between 1947 and 2003 [20]. In addition, Moore and McKendry [21] established regional snow accumulation anomalies and their relationship to atmospheric circulation patterns across BC. Snowpack evolution in the Quesnel River Basin of BC has been investigated using passive microwave remote sensing data; however, their application is limited by the region’s deep snowpacks that cause the remote sensing products to become saturated [22]. A study conducted by Shrestha et al. [23] reported declining contributions of snow in 21st century runoff projections in the Fraser River Basin. The latter findings are in accord with those of Morrison et al. [24] and Kerkhoven and Gan [25] where it is suggested that the Fraser River may transition from a snowmelt- to a rainfall-dominated regime by 2100. Thus, climate change is expected to alter SWE amounts and timing of the spring melt in the Upper Fraser River Basin, which may significantly impact its ecosystems including that of the ITR. Thus, the objective of this study is to quantify net snowpack accumulation and ablation characteristics across the Upper Fraser River Basin with a focus on the implications to the ecohydrology of its ITR. Some of the research questions motivating this effort are: (1) Does elevation and/or air temperature play(s) a dominant control on peak SWE accumulation in the study area?; (2) Is there consistent regional interannual variability in peak SWE values?; and (3) What may be the implications of observed changes in snowpack accumulation and ablation characteristics to the ecohydrology of the ITR, particularly in the context of climate change? To address these questions, daily SWE and air temperature data from seven snow pillow sites in the study area are used to explore snowpack characteristics such as annual peak accumulation and its timing, net accumulation and ablation rates, and duration with emphasis on their interannual variability. Basic information on these variables is needed to better understand the impacts of changing snow regimes on ITR dynamics and evolution. In particular, we investigate the sensitivity of peak SWE accumulation and its timing to air temperatures and then place this in the context of projected long-term regional warming and implications to old growth trees in the ITR. 2. Study Area The Upper Fraser River Basin of BC located upstream of the Nechako River spans ≈35,000 km2 at a mean elevation of 1413 m a.s.l., forming the headwaters of the Fraser River, the largest Canadian river

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flowing into the Pacific Ocean (Figure 1; [26]). The Upper Fraser River flows northwestward from the Rocky Mountains into the Rocky Mountain trench prior to veering southward near the city of Prince George, BC where its annual discharge rate is ≈26 km3·yr−1 [26]. The watershed’s highest peak is Mount Robson at 3954 m a.s.l. whereas its lowest level is in Prince George, BC at 575 m a.s.l. [27]. Mixed deciduous (aspen, willow and birch trees) and coniferous (lodgepole pine, subalpine fir and Engelmann spruce) forests cover the basin with meadows and rocky terrain in alpine areas. In addition, ancient western redcedar stands are typically found at toe slopes of the Cariboo and Rocky Mountains [3]. These old growth stands are part of the ITR that is characterized by abundant precipitation with a large fraction arriving as snowfall. Figure 1. (a) Map of British Columbia showing the study area (red rectangle) and the Fraser River Basin (blue outline) and (b) Location of the seven snow pillow sites in the Upper Fraser River Basin (blue line) used in the present study.

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The climate of the Upper Fraser River Basin is characterized by relatively cold, snowy winters as well as warm and occasionally wet summers throughout the entire basin. The 1971–2000 mean annual air temperature at Prince George, BC is 4.0 °C and total annual precipitation is 601 mm with 36% arriving in the form of snow (Environment Canada, 2014, https://weather.gc.ca/). Air temperatures decline with altitude, but precipitation generally increases at higher elevations, with greater fractions of the precipitation falling as snow. In the ITR, elevations range from 400 to 1500 m a.s.l., mean annual air temperatures range from 2.7 to 4.5 °C, and the mean annual precipitation is 788–1240 mm [3]. Snow covers the ground for about four months in the valley bottom of the basin and exhibits a linear increase in duration with elevation, with permanent snow fields and glaciers above ≈2400 m a.s.l. [28]. Frequent snowfalls lead to the development of a significant snowpack, with maximum SWE accumulations exceeding 800 mm at elevations above 1500 m a.s.l. [22]. The onset of snowmelt in spring induces an annual pulse of freshwater in the Upper Fraser River and its vast network of tributaries [29] making snowpack monitoring by the BC River Forecast Centre most essential. The Upper Fraser River Basin is important to the economy of western Canada with many active resource extraction industries (e.g., mining and forestry). Other important economic drivers in the basin include agriculture, recreation and tourism. The Fraser River is also one of the most productive salmon rivers in the world with up-river migrations in the millions each year [26,30]. Five species of salmon migrate and spawn in the Fraser River and its many tributaries, forming important resources for commercial and recreational fisheries as well as a source of sustenance for First Nations communities in the area. It is also a region where air temperatures have warmed by ≈1 °C since the early 1940s, possibly leading to warmer winters during which less precipitation will fall as snow that may in turn negatively impact snowpack levels [27]. There is thus an urgent need to better understand net snowpack accumulation and ablation characteristics in the ITR of the Upper Fraser River Basin. 3. Data and Methods Data from seven BC River Forecast Centre snow pillow sites in the Upper Fraser River Basin are used in the present study (Table 1). Data used include daily SWE as well as minimum and maximum air temperature. The period of data availability varies across sites and spans a minimum of seven winters with the earliest records beginning in 1968 at Barkerville, BC. The mean daily air temperature is estimated from the average of the daily minimum and maximum air temperatures. Data are then verified for completeness and gaps of seven days or less are in-filled through linear interpolation over time. Hydrological years (defined here as 1 September to 31 August of the following year since accumulation occasionally begins in September in the study area) with longer data gaps are eliminated from the analyses. Longer periods of missing air temperature data, often during summer, are in-filled with the daily mean for the same day over the period of record. A 5-day moving average of daily SWE and air temperature is then applied to filter out short-term fluctuations in these quantities arising from synoptic activity and possible instrumental errors (e.g., [31]). Several statistics are then compiled from the time series of 5-day moving averages of SWE and air temperature. This includes the mean and standard deviation of snow season length, snow onset and off dates, peak accumulation and timing, April 1 SWE, net accumulation and ablation rates, air temperature conditions during these periods, and melt factor (i.e., degree-day factor) over the sites’

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respective periods of data availability. Continuous snow cover was defined to occur for SWE values ≥20 mm. Degree day factors (a.k.a. melt coefficients with units of mm·°C−1·day−1) are the amounts of snowmelt that occurs per positive degree day and are thus assessed by tracking the air temperatures ≥0 °C at each site when SWE decreases [32]. Cycles of the mean daily SWE and air temperature at the seven sites over the course of a hydrological year are also presented. Cross correlations between time series of peak SWE values at each site for overlapping periods are also established. In addition, correlations between peak SWE values and air temperature conditions during the accumulation and ablation periods are computed. All correlation values are considered statistically-significant when p ≤ 0.05. Further analyses investigate the relationships between snowpack characteristics and geographical location (latitude, longitude and elevation). Trends (considered statistically-significant when p ≤ 0.05) in peak SWE values and their timing are then established using linear regressions for four sites (Barkerville, Revolution Creek, Yanks Peak East and Yellowhead) with the longest temporal records. Table 1. Information on the BC River Forecast Centre snow pillow stations (locations shown in Figure 1) used in this study. Station Name

Station I.D.

Latitude (°N)

Longitude (°W)

Elevation (m a.s.l.)

Mean Annual Peak SWE (mm)

Years of Data Availability

Barkerville Dome Mountain Hedrick Lake McBride (Upper) Revolution Creek Yanks Peak East Yellowhead

1A03P 1A19P 1A14P 1A02P 1A17P 1C41P 1A01P

53.05 53.62 54.10 53.30 53.78 52.82 52.90

121.48 121.02 121.00 120.32 120.37 121.35 118.53

1483 1768 1118 1608 1676 1683 1847

391.3 893.4 924.4 562.6 882.6 900.5 582.2

1968–20121 2005–2012 1999–2012 1971–20122 1984–2012 1996–2012 1997–2012

Note: 1Data from 29 November 1988 to 25 August 1996 are missing. 2Data from 4 June 1986 to 7 July 2006 are missing.

Even though the snow pillow data of SWE have some limitations (see Section 5.3), it is critical to assess SWE across the Upper Fraser Basin based on seven snow pillow sites over the past few decades. Thus, it allows for the mean characteristics, interannual variability, and long-term trends over the hydrological year to be established for this region. In addition, this study on the evolution of snowpack conditions in a very remote and undersampled region provides crucial information to begin understanding the ITR’s ecohydrology and dynamics in a highly variable and rapidly changing environment. 4. Results The mean hydrological year cycle of daily SWE exhibits smooth, nearly linear trends during the accumulation period (Figure 2). Accumulation of snow begins as air temperatures transition to subfreezing conditions (Figure 3). Once formed (SWE ≥ 20 mm), the snowpack remains continuous over time and is marked in individual hydrological years by rapid increases from snowfall events (not shown). Peak accumulation averages ≈750 mm SWE at the seven sites and typically occurs in mid- to late April when there is then a rapid decline in SWE until the termination of melt. Air temperatures approach 0 °C at peak accumulation and remain above this value during snow melt.

Hydrology 2014, 1 Figure 2. Hydrological year cycle of daily mean SWE at seven snow pillow sites in the Upper Fraser River Basin over their respective periods of data availability. For the plot of average SWE, the red, orange, green and blue vertical lines denote the snow onset date, April 1, day of maximum SWE, and snow offset date, respectively whereas the cyan and magenta horizontal lines denote the snow accumulation and ablation periods, respectively.

Figure 3. Hydrological year cycle of daily mean air temperature at seven snow pillow sites in the Upper Fraser River Basin over their respective periods of data availability.

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Table 2 provides overall statistics for snowpack characteristics at the seven sites of interest over their respective periods of record. Hydrological year maximum SWE varies between 391.3 mm at Barkerville and 924.4 mm at Hedrick Lake, with a large degree of interannual variability. Peak accumulation dates vary from 16 April at Barkerville to 12 May at Dome Mountain with standard deviations of about two weeks. Mean snow onset dates range from 12 October at Revolution Creek to 5 November at Barkerville, whereas mean snow off dates range from 30 May at Barkerville to 3 July at Dome Mountain. This implies the duration of snow cover extends from 206 to 258 days at the study sites. The period of net accumulation typically lasts three to four times longer than the period of net ablation. After peak accumulation, the mean net ablation rate varies between 8.8 and 21.7 mm·day−1 with a mean degree day factor ranging from 1.4 to 5.2 mm·day−1·°C−1. Table 2. Results of the SWE analyses. Variable Mean and [SD] (Units) Annual Maximum SWE (mm) Annual Day of Year of Maximum SWE (mm) April 1 Annual SWE (mm) Snow Onset Day

Snow Offset Day Duration of Snow Cover (days) Net Accumulation Period (days) Net Ablation Period (days) Ablation Rate (mm·day−1) Degree Day Factor (mm·day−1·°C−1)

Barkerville

Dome Mtn.

Hedrick Lake

McBride (Upper)

Revolution Creek

Yanks Peak East

Yellowhead

391.3 [79.7] 105.8 (16 Apr) [10.9] 363.9 [75.8] 309.2 (5 Nov) [13.1] 150.4 (30 May) [9.7] 206.4 [16.8]

893.4 [219.7] 131.7 (12 May) [5.1] 782.1 [208.5] 290.6 (18 Oct) [10.0] 183.9 (3 Jul) [9.4] 258.4 [9.2]

924.4 [205.8] 117.4 (27 Apr) [12.0] 851.7 [213.7] 305.8 (2 Nov) [13.6] 163.0 (12 Jun) [9.0] 222.4 [17.7]

562.6 [122.5] 115.9 (26 Apr) [7.0] 509.0 [110.8] 302.7 (30 Oct) [11.6] 163.0 (12 Jun) [6.8] 225.5 [14.3]

882.9 [247.1] 110.0 (20 Apr) [14.2] 829.8 [234.6] 285.3 (12 Oct) [16.0] 173.7 (23 Jun) [13.2] 253.6 [20.1]

900.5 [172.9] 121.0 (1 May) [14.0] 808.1 [144.4] 289.3 (16 Oct) [11.0] 170.9 (21 Jun) [15.1] 246.8 [17.3]

582.2 [132.6] 115.7 (25 Apr) [16.6] 507.7 [183.5] 288.3 (15 Oct) [9.1] 165.4 (14 Jun) [27.1] 242.3 [28.5]

161.9 [19.2]

206.3 [12.3]

176.8 [18.8]

178.4 [14.3]

189.9 [22.6]

196.9 [21.1]

192.7 [21.0]

44.6 [11.9] 8.8 [2.5] 2.2 [1.3]

52.1 [11.1] 16.7 [2.4] 2.8 [0.2]

45.6 [15.4] 21.7 [7.2] 4.0 [1.3]

47.1 [9.3] 12.0 [3.7] 5.2 [8.4]

63.7 [13.1] 13.8 [3.6] 3.5 [2.0]

49.9 [9.8] 18.3 [4.5] 2.4 [0.5]

49.7 [19.0] 16.9 [20.6] 1.4 [0.6]

The four sites with the longest time series of net snow accumulation show no recent trends in peak SWE (Figure 4a). There is less covariability between peak SWE at Yellowhead and values observed at the other sites owing in part to its more eastern and distant location in the Canadian Rockies (see Figure 1). The snow pillow site at Revolution Creek exhibits a statistically-significant trend toward a later date for the occurrence of peak accumulation (Figure 4b). Time series of snow onset dates, snow

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off dates, snow cover duration and mean air temperature during the accumulation period exhibit mixed trends (none however at p < 0.05) at the four sites with extended temporal coverage (Figure 4c,d). Peak SWE is significantly correlated with the date on which it occurs at Yanks Peak East (r = 0.66) and positively but not significantly correlated at other sites, partially explaining the tendency toward later occurrences of this event. A correlation analysis reveals no statistically-significant relationships between peak SWE and either latitude (r = 0.53), longitude (r = 0.19) or elevation (r = −0.11). The cross correlation analysis of peak SWE between sites shows a high degree of coherence (Table 3), indicating that wintertime synoptic activity affects the sites in a similar fashion. Figure 4. Hydrological year (a) peak SWE time series and linear trends; (b) day of peak SWE time series and linear trends; (c) snow onset days, snow off days, and snow cover duration time series; and (d) annual mean daily air temperature during the accumulation period time series and linear trends at four snow pillow sites in the Upper Fraser River Basin over their respective periods of data availability, 1990–2012.

(a)

(b)

(c)

(d)

Mean annual air temperatures at the seven sites of interest are all near 0 °C despite their 729 m range in elevations (Table 4). Snow onset days occur a week or two after the day when air temperatures fall below the freezing point whereas there is a 1–2 month delay in snow off dates after air temperatures rise above the melting point. Thus, the duration of the period with air

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temperatures